hi i had a recent problem. My iphone suddenly started to vibrate and it didnt stop. I was receiving calls, the line was ringing but my iphone wasnt physically. the screen was blank/black. no buttons i pressed mattered. it kept vibrating for 45 minutes until i i pressed the on/off and home button for 30sec to force it restart. why did this happen? and what causes does it affect the future? thnx.